In a communication transmission system, local oscillator feedthrough is a problem that needs to be highlighted. The local oscillator feedthrough is mainly caused by direct current components of two signals I/Q at an input end of a mixer. The local oscillator feedthrough worsens an error vector magnitude (EVM) of a desired signal, transmit power accuracy of a desired signal, spurious emission, linearity of a transmit link, and the like. Therefore, the local oscillator feedthrough needs to be processed.
In a typical application, a local oscillator filter is disposed at an output end of a mixer to filter out local oscillator feedthrough. However, in a microwave application, a filter for local oscillator feedthrough rejection is difficult to be implemented. In addition, for a direct up-conversion system, a local oscillator signal falls within a signal band, and cannot be filtered out by a filter.